Alloy
An alloy
is a
mixture or
metallic
solid
solution
composed
of two or
more
elements.
Complete
solid
solution
alloys
give
single
solid
phase
microstructure,
while
partial
solutions
give two
or more
phases
that may
or may not
be
homogeneous
in
distribution,
depending
on thermal
(heat
treatment)
history.
Alloys
usually
have
different
properties
from those
of the
component
elements.
Alloy
constituents
are
usually
measured
by mass.

Theory
Alloying a
metal is
done by
combining
it with
one or
more other
metals or
non-metals
that often
enhance
its
properties.
For
example,
steel is
stronger
than iron,
its
primary
element.
The
physical
properties,
such as
density,
reactivity,
Young's
modulus,
and
electrical
and
thermal
conductivity,
of an
alloy may
not differ
greatly
from those
of its
elements,
but
engineering
properties
such as
tensile
strength
and shear
strength
may be
substantially
different
from those
of the
constituent
materials.
This is
sometimes
a result
of the
sizes of
the atoms
in the
alloy,
because
larger
atoms
exert a
compressive
force on
neighboring
atoms, and
smaller
atoms
exert a
tensile
force on
their
neighbors,
helping
the alloy
resist
deformation.
Sometimes
alloys may
exhibit
marked
differences
in
behavior
even when
small
amounts of
one
element
occur. For
example,
impurities
in
semi-conducting
ferromagnetic
alloys
lead to
different
properties.
Some
alloys are
made by
melting
and mixing
two or
more
metals.
Bronze, an
alloy of
copper and
tin, was
the first
alloy
discovered,
during the
prehistoric
period now
known as
the bronze
age; it
was harder
than pure
copper and
originally
used to
make tools
and
weapons,
but was
later
superseded
by metals
and alloys
with
better
properties.
In later
times
bronze has
been used
for
ornaments,
bells,
statues,
and
bearings.
Brass is
an alloy
made from
copper and
zinc.

Solidus -
Liquidus -
Eutectic
Unlike
pure
metals,
most
alloys do
not have a
single
melting
point, but
a melting
range in
which the
material
is a
mixture of
solid and
liquid
phases.
The
temperature
at which
melting
begins is
called the
solidus,
and the
temperature
when
melting is
just
complete
is called
the
liquidus.
However,
for most
alloys
there is a
particular
proportion
of
constituents
(in rare
cases
two)—the
eutectic
mixture—which
gives the
alloy a
unique
melting
point.

Metallurgy:
Making
Alloys

The
majority
of alloys
are
prepared
by mixing
metals in
the molten
state;
then the
mixture is
poured
into metal
or sand
moulds and
allowed to
solidify.
Generally
the major
ingredient
is melted
first;
then the
others are
added to
it and
should
completely
dissolve.
For
instance,
if a
plumber
makes
solder he
may melt
his lead,
add tin,
stir, and
cast the
alloy into
stick
form. Some
pairs of
metals do
not
dissolve
in this
way. When
this is so
it is
unlikely
that a
useful
alloy will
be formed.
Thus if
the
plumber
were to
add
aluminum,
instead of
tin, to
the lead,
the two
metals
would not
dissolve -
they would
behave
like oil
and water.
When cast,
the metals
would
separate
into two
layers,
the heavy
lead below
and
aluminum
above.

One
difficulty
in making
alloys is
that
metals
have
different
melting
points.
Thus
copper
melts at
1,083 C,
while zinc
melts at
419 C and
boils at
907 C So,
in making
brass, if
we just
put pieces
of copper
and zinc
in a
crucible
and heated
them above
1,083 C,
both the
metals
would
certainly
melt. But
at that
high
temperature
the liquid
zinc would
also boil
away and
the vapor
would
oxidize in
the air.
The method
adopted in
this case
is to heat
first the
metal
having the
higher
melting
point,
namely the
copper.
When this
is molten,
the solid
zinc is
added and
is quickly
dissolved
in the
liquid
copper
before
very much
zinc has
boiled
away. Even
so, in the
making of
brass,
allowance
has to be
made for
unavoidable
zinc loss
which
amounts to
about one
part in
twenty of
the zinc.
Consequently,
in
weighing
out the
metals
previous
to
alloying,
an extra
quantity
of zinc
has to be
added.

Sometimes
the making
of alloys
is
complicated
because
the higher
melting
point
metal is
in the
smaller
proportion.
For
example,
one light
alloy
contains
92 per
cent
aluminum
(melting
point 660
C) with 8
per cent
copper
(melting
point
1,083 C).
To
manufacture
this alloy
it would
be
undesirable
to melt
the few
pounds of
copper and
add nearly
twelve
times the
weight of
aluminum.
The metal
would have
to be
heated so
much to
persuade
the large
bulk of
aluminum
to
dissolve
that gases
would be
absorbed,
leading to
unsoundness.
In this,
as in many
other
cases, the
alloying
is done in
two
stages.
First an
intermediate
'hardener
alloy' is
made,
containing
50 per
cent
copper and
50 per
cent
aluminum,
which
alloy has
a melting
point
considerably
lower than
that of
copper
and, in
fact,
below that
of
aluminum.
Then the
aluminum
is melted
and the
correct
amount of
the
hardener
alloy
added;
thus, to
make l00lb
of the
aluminum-copper
alloy we
should
require
84lb. of
aluminum
to be
melted
first and
16lb of
hardener
alloy to
be added
to it.

In a few
cases, the
melting
point of
the alloy
can be
worked out
approximately
by
arithmetic.
For
instance,
if copper
(melting
point
1,083 C)
is alloyed
with
nickel
(melting
point
1,454 C) a
fifty-fifty
alloy will
melt at
about
halfway
between
the two
temperatures.
Even in
this case
the
behavior
of the
alloy on
melting is
not
simple. A
copper-nickel
alloy does
not melt
or freeze
at one
fixed and
definite
temperature,
but
progressively
solidifies
over a
range of
temperature.
Thus, if a
fifty-fifty
copper-nickel
alloy is
liquefied
and then
gradually
cooled, it
starts
freezing
at 1,312
C, and as
the
temperature
falls,
more and
more of
the alloy
becomes
solid
until
finally at
1,248 C it
has
completely
solidified.
Except in
certain
special
cases this
'freezing
range'
occurs in
all
alloys,
but it is
not found
in pure
metals,
metallic,
or
chemical
compounds,
and in
some
special
alloy
compositions,
referred
to below,
all of
which melt
and freeze
at one
definite
temperature.

The
alloying
of tin and
lead
furnishes
an example
of one of
these
special
cases.
Lead melts
at 327 C
and tin at
232 C. If
lead is
added to
molten tin
and the
alloy is
then
cooled,
the
freezing
point of
the alloy
is found
to be
lower than
the
freezing
points of
both lead
and tin
(see
figure 1).
For
instance,
if a
molten
alloy
containing
90 per
cent tin
and 10 per
cent lead
is cooled,
the
mixture
reaches a
temperature
of 217 C
before it
begins to
solidify.
Then, as
the alloy
cools
further,
it
gradually
changes
from a
completely
fluid
condition,
through a
stage when
it is like
gruel,
until it
becomes as
thick as
porridge,
and
finally,
at a
temperature
as low as
183 C, the
whole
alloy has
become
completely
solid. By
referring
to figure
1, it can
be seen
that with
80 per
cent tin,
the alloy
starts
solidifying
at 203 C,
and
finishes
only when
the
temperature
has fallen
to 183 C
(note the
recurrence
of the 183
C).

What
happens at
the other
end of the
series,
when tin
is added
to lead?
Once again
the
freezing
point is
lowered.
An alloy
with only
20 per
cent tin
and the
remainder
lead
starts to
freeze at
279 C and
completes
solidification
at the now
familiar
temperature
of 183 C.
One
particular
alloy,
containing
62 per
cent tin
and 38 per
cent lead,
melts and
solidifies
entirely
at 183 C.
Obviously
this
temperature
of 183 C
and the
62/38 per
cent
composition
are
important
in the
tin-lead
alloy
system.
Similar
effects
occur in
many other
alloy
systems
and the
special
composition
which has
the lowest
freezing
point of
the series
and which
entirely
freezes at
that
temperature
has been
given a
special
name. The
particular
alloy is
known as
the
'eutectic'
alloy and
the
freezing
temperature
(183 C in
the case
of the
tin-lead
alloys) is
called the
eutectic
temperature.

By a
careful
choice of
constituents,
it is
possible
to make
alloys
with
unusually
low
melting
points.
Such a
fusible
alloy is a
complex
eutectic
of four or
five
metals,
mixed so
that the
melting
point is
depressed
until the
lowest
melting
point
possible
from any
mixture of
the
selected
metals is
obtained.
A familiar
fusible
alloy,
known as
Wood's
metal, has
a
composition:

Bismuth -
4 parts
Lead - 2
parts
Tin - 1
part
Cadmium -
1 part

and its
melting
point is
about 158F
/ 70 C;
that is,
less than
the
boiling
point of
water.
Practical
jokers
have
frequently
amused
themselves
by casting
this
fusible
alloy into
the shape
of a
teaspoon,
which will
melt when
used to
stir a cup
of hot
tea.
* Warning:
Notice the
TOXIC
Metals?

These low
melting
point
alloys are
regularly
in use for
more
serious
purposes,
as for
example,
in
automatic
anti-fire
sprinklers
installed
in the
ceilings
of
buildings.
Each jet
of the
water
sprinkler
system
contains a
piece of
fusible
alloy, so
that if a
fire
occurs and
the
temperature
rises
sufficiently
high, the
alloy
melts and
the water
is
released
through
the jets
of the
sprinkler.

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